Variable focal length optical lenses are of great value, particularly in the area of eyeglasses or spectacles. The human eye, after a person reaches the mid-forties, loses its natural flexibility for focusing on both near and far objects. As a consequence, most people require spectacles to assist them for both reading and long-distance vision. Several products are available for this purpose: reading spectacles, bifocal spectacles, and multifocal spectacles; but each has its drawbacks. Reading spectacles must be put on before beginning to read, assuming they can be found when needed. Bifocals allow close distance viewing, but only at restricted low angles which generally require the wearer to tilt his head to an unnatural position. Multifocal spectacles allow correction for a continuum of distances, but an even sharper angular restriction is involved and constant tilting of the head is required to focus on an object at a given distance. Moreover, some people experience difficulties in adjusting to bifocals and multifocal spectacles.
Spectacles with variable power lenses in which the focal length can be changed in response to the touch of a switch, or movement of a lever on or near the frame of the glasses are obviously of potential advantage to a user. U.S. Pat. Nos. 4,261,655 and 4,418,990 disclose fluid-filled flexible lenses which can be adjusted to provide variable power. The flexibility of the materials and the fluid in these lenses permit controlled variation in their focal lengths. One limitation in this approach, however, is that the optical quality of the lens is not preserved during focal variations. Failure to maintain the optical quality of a lens results in distortion that is usually unacceptable to a wearer. The problem arises because the shape variation of the surfaces of the lenses do not remain substantially spherical during focal variations. Ideally, the outer surface of a lens should be spherical before a focal change takes place, it should remain spherical after such change.
An object of the present invention, therefore, is to provide a new and improved variable power lens that solves the problems described above and achieves other advantages for spectacles as well as for magnifying glasses, or lenses or mirrors within various optical instruments (e.g., variable focus ocular lenses in a microscope).